Optical fiber

ABSTRACT

An optical fiber includes: a core portion made of glass; and a cladding portion made of glass, having a refractive index lower than the refractive index of the core portion, and positioned on an outer periphery of the core portion. Further, the cladding portion has an outer diameter smaller than 100 μm, and the core portion has a relative refractive-index difference of 0.32% to 0.40% with respect to the cladding portion.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.17/011,665, filed Sep. 3, 2020, which is a continuation of InternationalApplication No. PCT/JP2019/007869, filed on Feb. 28, 2019 which claimsthe benefit of priority of the prior Japanese Patent Application No.2018-041711, filed on Mar. 8, 2018, the entire contents of which areincorporated herein by reference.

BACKGROUND

The present disclosure relates to an optical fiber.

In the datacom and telecom fields, a small-diameter optical fiber hasattracted attention as an optical fiber that achieves a high-densityoptical fiber cable. In the related art, a configuration in which a coreportion has a high relative refractive-index difference with respect toa cladding portion has been disclosed for the small-diameter opticalfiber (Murase et al., “Development of small-diameter clad fiber”, ShowaElectrical Wire review, vol. 53, No. 1 (2003), pp. 32-36). In addition,a configuration in which a trench layer is provided adjacent to the coreportion has been disclosed (International Publication No. WO2016/190297).

SUMMARY

There is a need for providing a small-diameter optical fiber having areduced leakage loss.

According to an embodiment, an optical fiber includes: a core portionmade of glass; and a cladding portion made of glass, having a refractiveindex lower than the refractive index of the core portion, andpositioned on an outer periphery of the core portion. Further, thecladding portion has an outer diameter smaller than 100 μm, and the coreportion has a relative refractive-index difference of 0.32% to 0.40%with respect to the cladding portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an optical fiber accordingto a first embodiment;

FIG. 2A is a diagram illustrating the refractive index profile of theoptical fiber illustrated in FIG. 1 ;

FIG. 2B is a diagram illustrating the refractive index profile of theoptical fiber illustrated in FIG. 1 ;

FIG. 3 is a diagram illustrating the relation of a threshold fiberdiameter with Δ1 and a core diameter;

FIG. 4 is a diagram illustrating the refractive index profile of anoptical fiber according to a second embodiment;

FIG. 5 is a diagram illustrating the relation of chromatic dispersionand MFD with the distance between a trench layer and a core portion;

FIG. 6 is a diagram illustrating the relation of an MFD change amountand a cutoff wavelength change amount with Δ3 when the core portion andthe trench layer are adjacent to each other;

FIG. 7 is a diagram illustrating a measurement result of a transmissionloss;

FIG. 8A is a diagram illustrating the relation of a zero-dispersionwavelength with a trench position;

FIG. 8B is a diagram illustrating the relation of a dispersion slopewith the trench position;

FIG. 8C is a diagram illustrating the relation of the zero-dispersionwavelength with the trench position;

FIG. 8D is a diagram illustrating the relation of the dispersion slopewith the trench position;

FIG. 9A is a diagram illustrating the relation of the zero-dispersionwavelength with the trench position;

FIG. 9B is a diagram illustrating the relation of the dispersion slopewith the trench position;

FIG. 9C is a diagram illustrating the relation of the zero-dispersionwavelength with the trench position;

FIG. 9D is a diagram illustrating the relation of the dispersion slopewith the trench position;

FIG. 10A is a diagram illustrating the relation of a bending loss withthe trench position;

FIG. 10B is a diagram illustrating the relation of a threshold fiberwith the trench position;

FIG. 10C is a diagram illustrating the relation of the bending loss withthe trench position;

FIG. 10D is a diagram illustrating the relation of the threshold fiberwith the trench position;

FIG. 11A is a diagram illustrating the relation of the bending loss withthe trench position;

FIG. 11B is a diagram illustrating the relation of the threshold fiberwith the trench position;

FIG. 11C is a diagram illustrating the relation of the bending loss withthe trench position;

FIG. 11D is a diagram illustrating the relation of the threshold fiberwith the trench position;

FIG. 12A is a diagram illustrating the relation of the MFD with thetrench position;

FIG. 12B is a diagram illustrating the relation of a cutoff wavelengthwith the trench position;

FIG. 12C is a diagram illustrating the relation of the MFD with thetrench position;

FIG. 12D is a diagram illustrating the relation of the cutoff wavelengthwith the trench position;

FIG. 13A is a diagram illustrating the relation of the MFD with thetrench position;

FIG. 13B is a diagram illustrating the relation of the cutoff wavelengthwith the trench position;

FIG. 13C is a diagram illustrating the relation of the MFD with thetrench position;

FIG. 13D is a diagram illustrating the relation of the cutoff wavelengthwith the trench position;

FIG. 14 is a diagram illustrating the relation of the MFD with thetrench position; and

FIG. 15 is a diagram illustrating change of the threshold fiber diameterwhen parameters are changed.

DETAILED DESCRIPTION

In the related art, characteristics of an optical fiber disclosed inMurase et al., “Development of small-diameter clad fiber”, ShowaElectrical Wire review, vol. 53, No. 1 (2003), pp. 32-36 do not satisfy,for example, a typical single-mode optical-fiber standard (hereinafterreferred to as G.650.2 standard) defined by ITU-T (InternationalTelecommunication Union) G.650.2. In addition, characteristics of anoptical fiber disclosed in International Publication No. WO 2016/190297satisfy G.650.2 standard but the outer diameter (fiber diameter) of thecladding portion is 100 μm to 125 μm approximately, which isinsufficient for diameter reduction that will be further increasinglyrequired. In addition, a leakage loss increases as the diameter of anoptical fiber is reduced, but no optical fiber having a leakage lossreduced to a sufficiently small value and having a small diameter hasbeen disclosed.

Embodiments of the present disclosure will be described below in detailwith reference to the accompanying drawings. In addition, the presentdisclosure is not limited by the embodiments described below. In thedrawings, components identical or corresponding to each other aredenoted by an identical reference sign as appropriate. In addition, inthe present specification, a cutoff wavelength is a cable cutoffwavelength defined by ITU-T G.650.1. In addition, other terms notparticularly defined in the present specification are subjected todefinitions and measurement methods in G.650.1.

First Embodiment

FIG. 1 is a schematic cross-sectional view of an optical fiber accordingto a first embodiment. This optical fiber 10 includes a core portion 11positioned substantially at the center of the optical fiber 10, and acladding portion 12 positioned on the outer periphery of the coreportion 11. The cladding portion 12 includes an adjacent region 12 aadjacent to and surrounding the outer periphery of the core portion 11,and a non-adjacent region 12 b positioned on the outer periphery of theadjacent region 12 a and adjacent to and surrounding the outer peripheryof the adjacent region 12 a. In other words, the adjacent region 12 a isinterposed between the core portion 11 and the non-adjacent region 12 b.

The core portion 11 and the cladding portion 12 are each made ofsilica-based glass. For example, the core portion 11 is made of silicaglass to which a dopant such as germanium (Ge) is added to increase therefractive index. The cladding portion 12 has a refractive index lowerthan the refractive index of the core portion 11. In addition, in theoptical fiber 10, the adjacent region 12 a and the non-adjacent region12 b of the cladding portion 12 are each made of pure silica glasscontaining no refractive index adjustment dopants such as Ge andfluorine (F).

Note that a coating made of, for example, resin is formed to cover theouter periphery of the cladding portion 12 when the optical fiber 10 isused. The coating is made of, for example, UV curable resin and has alayer structure of one layer or two or more layers. The UV curable resinis, for example, urethane acrylate series, polybutadiene acrylateseries, epoxy acrylate series, silicone acrylate series, or polyesteracrylate series, but is not particularly limited as long as it is usedfor optical-fiber coating. In addition, in the optical fiber 10, focusis on diameter reduction of the core portion 11 and the cladding portion12 as glass parts, and thus the outer diameter of the cladding portion12 is defined to be a fiber diameter as a diameter reduction target.

FIGS. 2A and 2B are each a diagram illustrating a refractive indexprofile in the radial direction from the central axis of the opticalfiber 10. In FIG. 2A, a profile P11 illustrates the refractive indexprofile of the core portion 11, and a profile P12 illustrates therefractive index profile of the cladding portion 12. Note that eachrefractive index profile is indicated as a relative refractive-indexdifference with respect to the cladding portion 12. As illustrated inFIG. 2A, the optical fiber 10 has a step-type refractive index profile,the diameter (core diameter) of the core portion 11 is 2 a, and therelative refractive-index difference of the core portion 11 with respectto the cladding portion 12 is Δ1.

Note that, as illustrated in FIG. 2B, the refractive index profile ofthe core portion 11 is not always of a step type having a geometricallyideal shape, but the shape of a top part is not flat and hasirregularities formed due to manufacturing characteristics or has askirt from the top part in some cases. In such a case, the value of atleast part of the top part of the refractive index profile in the rangeof the core diameter 2 a in manufacturing design is an index thatdetermines Δ1.

Subsequently, designing for reduction of the diameter of the opticalfiber 10 will be described below. In the optical fiber 10, an opticalproperty as a restriction on diameter reduction is a leakage loss due tolight leakage. A structure that is dominant to the leakage loss andother characteristics is the structure of the core portion 11. Thus, aminimum fiber diameter (threshold fiber diameter) necessary forobtaining a leakage loss equal to or smaller than 0.001 dB/km at awavelength of 1625 nm was studied through simulation calculation bychanging the relative refractive-index difference Δ1 and the corediameter 2 a of the core portion 11. The leakage loss may be measured asa difference in the transmission loss at 1625 nm between an opticalfiber having a clad diameter that is large enough to avoid an increasein the leakage loss and an optical fiber having an actual fiberdiameter, both optical fibers having the same profile.

The study was made on an optical fiber that satisfies standards (1) to(5) according to G.652 standard, as follows: (1) have a mode fielddiameter (MFD) of 8.6 μm to 9.5 μm at a wavelength of 1310 nm; (2) havea bending loss equal to or smaller than 5.3×10⁻³ dB/m at a wavelength of1550 nm when bent at a diameter of 60 mm; (3) have a zero-dispersionwavelength of 1300 nm to 1324 nm; (4) have a dispersion slope equal toor smaller than 0.092 ps/nm²/km; and (5) have a cable cutoff wavelengthequal to or shorter than 1260 nm at the zero-dispersion wavelength.

FIG. 3 is a diagram illustrating the relation of the threshold fiberdiameter with Δ1 and the core diameter. As illustrated in FIG. 3 , itcan be understood that there is a condition that the leakage loss can besufficiently reduced by optimizing Δ1 and the core diameter asparameters of the core portion 11 when the fiber diameter is smallerthan the threshold fiber diameter of 100 μm. Accordingly, it is possibleto satisfy G.652 standard and also achieve diameter reduction to a fiberdiameter smaller than 100μ as compared to a optical fiber in the relatedart. Specifically, a small-diameter optical fiber 10 satisfying G.652standard, having a reduced leakage loss, and having a fiber diametersmaller than 100 μm can be achieved by having Δ1 of 0.32% to 0.40% and acore diameter of 7 μm to 10 μm. The fiber diameter smaller than 100 μmis significantly smaller than 125 μm, which is the fiber diameter of atypical optical fiber. Note that the coating structure (properties andthickness) and the like are not particularly limited but, for example,as publicly known, it is preferable to use a coating structure and thelike appropriately set to reduce a microbending loss and the like.

In addition, as described above, the top part of the refractive indexprofile of the core portion 11 of the optical fiber 10 does notnecessarily has a flat shape, but leakage-loss and small-diametercharacteristics can be obtained when at least part of the top part,specifically, a region of the top part determines that Δ1 has a relativerefractive-index difference of 0.32% to 0.40%. More preferably, a regionof 50% or more of the top part has a relative refractive-indexdifference of 0.32% to 0.40%. In addition, the relative refractive-indexdifference of the top part in the range of the core diameter 2 a inmanufacturing design more preferably has an average value, a maximumvalue, and a minimum value of 0.32% to 0.40% to obtain desiredcharacteristics.

In addition, influence of the coating and influence of the microbendingloss and the like occur in addition to influence of the leakage losswhen actual manufacturing of a cable of the optical fiber 10 is takeninto consideration. Thus, it is preferable to have a design with which90 μm or smaller with an allowance of 10% approximately to 100 μm can beachieved as the fiber diameter. It is preferable from the results ofFIG. 2 that the core portion 11 has Δ1 of 0.335% to 0.375% and a corediameter of 8.2 μm to 9.2 μm or has Δ1 of 0.37% to 0.395% and a corediameter of 7.6 μm to 8.3 μm.

Second Embodiment

Similarly to the optical fiber 10 according to the first embodiment, anoptical fiber according to a second embodiment includes a core portionmade of silica glass and a cladding portion made of silica-based glass,and the cladding portion includes an adjacent region and a non-adjacentregion. In addition, a coating is formed to cover the outer periphery ofthe cladding portion.

FIG. 4 is a diagram illustrating the refractive index profile of theoptical fiber according to the second embodiment in the radial directionfrom the central axis. In FIG. 4 , a profile P21 illustrates therefractive index profile of the core portion, and a profile P22illustrates the refractive index profile of the cladding portion. Notethat each refractive index profile is indicated as a relativerefractive-index difference 4.

As indicated by the profile P22, the cladding portion includes threelayers having profiles P22 a, P22 b, and P22 c, respectively. Theprofile P22 a is the refractive index profile of the adjacent region,and the adjacent region is made of pure silica glass. The profile P22 bis the refractive index profile of a layered region to which a dopantsuch as F is added to decrease the refractive index in the non-adjacentregion and that concentrically surrounds the core portion. This regionis referred to as a trench layer. The profile P22 c is the refractiveindex profile of a region positioned on the outer periphery of thetrench layer in the non-adjacent region and the outer diameter of whichdetermines the fiber diameter. This region is made of pure silica glassand is a reference refractive index region.

As illustrated in FIG. 4 , the optical fiber according to the secondembodiment has a trench-type refractive index profile, the core diameterof the core portion is 2 a, and the relative refractive-index differenceof the core portion with respect to the adjacent region is Δ1. Therelative refractive-index difference of the adjacent region with respectto the reference refractive index region is Δ2, which is 0% in thepresent embodiment. The relative refractive-index difference of thetrench layer with respect to the reference refractive index region isΔ3. In addition, the outer diameter of the adjacent region, in otherwords, the inner diameter of the trench layer is 2 b, and the outerdiameter of the trench layer is 2 c.

The optical fiber according to the second embodiment has a structure inwhich the trench layer is provided to the optical fiber 10 according tothe first embodiment, and preferable ranges of the values of Δ1 and thecore diameter are same as those of the optical fiber 10. Accordingly,similarly to the optical fiber 10, the optical fiber according to thesecond embodiment is a small-diameter optical fiber having a reducedleakage loss and can have a reduced macrobending loss (bending loss)because of the effect of the trench layer.

However, similarly to the optical fiber 10, in the optical fiberaccording to the second embodiment, the relative refractive-indexdifference Δ3 of the trench layer and the inner diameter 2 b and theouter diameter 2 c of the trench layer have preferable ranges to satisfyG.652 standard or satisfy G.657 standard, which requires a smallermacrobending loss. For example, b/a is preferably equal to or largerthan two for the core diameter 2 a, and b/a is preferably equal to orlarger than three. In addition, the width of the trench layer, (c−b), ispreferably 0.2 to 1 time larger than a, and Δ3 is preferably equal to orsmaller than −0.4%. These will be described later.

FIG. 5 is a diagram illustrating results of simulation calculation ofthe relation of chromatic dispersion and the MFD with the distancebetween the trench layer and the core portion in an optical fiberincluding the trench layer as in the second embodiment. Here, thedistance between the trench layer and the core portion is the distance(b−a) between the outer periphery of the core portion and the innerperiphery of the trench layer, and is a value corresponding to thethickness of the adjacent region. Note that the core diameter 2 a is 10μm, Δ3 is −1%, and the width of the trench layer, (c−b), is 10 μm, whichis equal to the core diameter 2 a. Note that the distance between thetrench layer and the core portion being 0 μm in FIG. 5 corresponds to acase in which the trench layer is adjacent to the core portion (in otherwords, the trench layer exists in the adjacent region). In addition, “notrench” corresponds to a case of the optical fiber 10 according to thefirst embodiment.

As illustrated in FIG. 5 , it can be understood that change of thechromatic dispersion and the MFD from the case of no trench is largewhen the distance between the trench layer and the core portion is 0 μm(in other words, the core portion and the trench layer are adjacent toeach other), but a stable state that has almost no change of thechromatic dispersion and the MFD is reached when the distance betweenthe trench layer and the core portion is equal to or longer than 10 μmapproximately, in other words, b/a is equal to or larger than threeapproximately. In addition, it can be said that change of the chromaticdispersion and the MFD becomes gradual when the distance between thetrench layer and the core portion reaches 5 μm approximately, in otherwords, b/a reaches 2. Thus, to reduce characteristic change from theoptical fiber 10 due to provision of the trench layer, the trench layeris preferably provided in the non-the adjacent region, b/a is preferablyequal to or larger than two, more preferably equal to or larger thanthree, when Δ3 is large at −1% approximately.

However, the present disclosure is not limited thereto, but the coreportion and the trench layer may be adjacent to each other as amodification of the second embodiment. FIG. 6 is a diagram illustratingresults of simulation calculation of the relation of an MFD changeamount (solid line) and a cutoff wavelength change amount (dashed line)with Δ3 when the core portion and the trench layer are adjacent to eachother. The core diameter and Δ1 are same as those in the case of FIG. 5. In addition, the width of the trench layer, (c−b), is 4 a. Note thatthese change amounts are calculated by changing the core diameter tothree values so that the cutoff wavelength is 1160 nm to 1260 nm. It canbe understood from FIG. 6 that the MFD change amount and the cutoffwavelength change amount are sufficiently small when the width of thetrench layer, (c−b), is relatively large at 4 a but Δ3 is equal to orlarger than −0.1% approximately and smaller than 0%. Note that thechange amounts can be smaller when the width of the trench layer, (c−b),is smaller than 4 a, but in this case, Δ3 is preferably set to be equalto or larger than −0.1% and smaller than 0% in accordance with allowedchange amounts of the MFD, the cutoff wavelength, and the like.

The optical fibers according to the first and second embodiments and themodification above can be configured to satisfy G.652 standard. Inaddition, when an optical fiber satisfying ITU-T G.654 standard (what iscalled a cutoff shift fiber, and for example, the cutoff wavelength isdefined to be equal to or shorter than 1530 nm) or G.657 standard (whatis called a bending resistance fiber), which require characteristicsclose to those of G.652 standard, is designed, it is favorable to employthe refractive index profile of the core portion of each optical fiberaccording to the first embodiment or 2 or the modification.

In addition, each optical fiber according to the first embodiment or 2or the modification, which has a small diameter, has a small amount of aglass material of the cladding portion, and thus is an optical fiber,the material cost of which is reduced.

The present disclosure will be further described below. Each opticalfiber according to the present disclosure is a small-diameter opticalfiber having a reduced leakage loss but preferably satisfies G.652standard as described above in practical use. Specifically, it ispreferable to achieve a leakage loss equal to or smaller than 0.001dB/km at a wavelength of 1625 nm and satisfy G.652A, G.652B, G.652C,G.652D standards listed in Table 1. Note that the macrobending loss hasa value at a wavelength of 1550 nm in G.652A standard but has a value ata wavelength of 1625 nm in G.652B, G.652C, and G.652D standards. Notethat, in this and following tables, “E” represents a power of 10, andfor example, “5.3E-3” means “5.3×10⁻³”.

TABLE 1 Zero- dispersion Dispersion Cutoff Macrobending wavelength slopeMFD wavelength loss [nm] [ps/nm²/km] [μm] [nm] @ 60 @ zero- @ 1310 @ 22m mm dispersion nm @ 1550 nm wavelength (1625 nm for B, C, D) 1300-≤0.092 8.6- ≤1260 ≤5.3E−3 1324 9.5

Table 2 lists exemplary results of simulation calculation ofcharacteristics of the optical fiber 10 according to the firstembodiment in which Δ1 is set to be 0.32% to 0.40% and the core diameter2 a is set to be 7 μm to 10 μm. In any case, G.652A standard issatisfied and the threshold fiber diameter is smaller than 100 μm. Inparticular, it can be understood that the fiber diameter can be 80 μmwhen Δ1 is 0.37% and 2 a is 9.0 μm. In other words, an optical fiberhaving Δ1 and a core diameter of these values has a fiber diametersmaller than 100 μm and has small influence of the leakage loss in awavelength band equal to or longer than the cutoff wavelength and equalto or shorter than 1625 nm in which single mode operation is performed.

TABLE 2 Dispersion Zero- slope Cutoff Macrobending Threshold Δ1 2adispersion [ps/nm²/km] MFD wavelength loss fiber [%] [μm] wavelength @zero-dispersion [μm] [nm] [db/m @ 60 mm] diameter G.652 A [nm]wavelength @ 1310 nm @ 22m @ 1550 nm [μm] standard 1300-1324 ≤0.0928.6-9.5 ≤1260 ≤5.3E−3  (<100) 0.39 8.0 1319 0.085 8.6 1145 3.9E−6 840.38 8.0 1320 0.085 8.7 1130 1.6E−5 86 0.37 8.0 1321 0.085 8.7 11186.2E−5 89 0.37 8.5 1311 0.087 8.9 1185 5.1E−6 84 0.37 9.0 1304 0.090 9.11254 3.3E−7 80 0.36 8.0 1322 0.084 8.8 1100 2.4E−4 92 0.36 9.0 13050.090 9.2 1241 1.8E−6 83 0.35 8.0 1323 0.084 8.9 1085 9.3E−4 96 0.35 9.01305 0.090 9.3 1222 8.5E−6 85 0.34 9.0 1306 0.089 9.4 1204 3.2E−5 880.33 9.0 1307 0.089 9.4 1185 1.4E−4 91

Note that, in an actual optical fiber, because of additional factorssuch as the microbending loss, it is important to use parameters listedin Table 2 and further use a coating structure having optimum resinproperty and thickness to reduce the microbending loss.

Here, an optical fiber as an example of the present disclosure wasmanufactured by a publicly known method to investigate matching of thesimulation results in Table 2 and characteristics of an actual opticalfiber. In the optical fiber of the example, Δ1 was 0.37%, the corediameter was 8.5 μm, and the fiber diameter was 90 μm. Note that a resinmaterial used in a normal optical fiber that satisfies G.650.2 standardwas used as the resin material of the coating, but a primary layer and asecondary layer were thinner than normal with outer diameters of 100 μmand 125 μm, respectively.

FIG. 7 is a diagram illustrating a measurement result of a transmissionloss of the optical fiber of the example. As illustrated in FIG. 7 ,loss increase was not observed on the long-wavelength side in atransmission loss spectrum of the optical fiber of the example. Thismeans that almost no leakage loss occurs in the optical fiber of theexample. In addition, as listed in Table 3, a calculation example thatis a simulation result and an actual value of the example match eachother at high accuracy.

TABLE 3 Trans- mission Cutoff Macrobending loss MFD wavelength loss Unit[dB/km] [μm] [nm] [dB/m @ 20 @ 1550 mm] nm @ 1550 nm Calculation — 10.01185 2.8 example Example 0.228 10.0 1195 2.1

Subsequently, it will be described based on a simulation result that thediameter of the optical fiber according to the second embodiment can befurther reduced. Hereinafter, simulation calculation is performed whileΔ1 is fixed to 3.6% but the core diameter 2 a, a trench position, atrench width, Δ3, and the like are changed. Here, the trench position isb/a. In addition, the trench width is a value normalized with the corediameter, in other words, the trench width is (c−b)/a.

First, simulation calculation was performed while the core diameter 2 awas 9 μm, Δ3 was −0.4%, −0.6%, −0.7%, −1.0%, and the trench position andthe trench width were changed.

FIGS. 8A to 8D and 9A to 9D are diagrams illustrating the relation ofdispersion characteristics, in other words, the zero-dispersionwavelength and the dispersion slope with the trench position. Thedispersion slope has a value at the zero-dispersion wavelength. FIGS. 8Aand 8B correspond to a case in which Δ3 is −0.4%, and FIGS. 8C and 8Dcorrespond to a case in which Δ3 is −0.6%. In addition, FIGS. 9A and 9Bcorrespond to a case in which Δ3 is −0.7%, and FIGS. 9C and 9Dcorrespond to a case in which Δ3 is −1.0%.

FIGS. 10A to 10D and 11A to 11D are diagrams illustrating the relationof optical confinement characteristics, in other words, the bending loss(macrobending loss) and the threshold fiber diameter with the trenchposition. The bending loss has a value at a wavelength of 1550 nm whenbending is made at a diameter of 60 mm. FIGS. 10A and 10B correspond toa case in which Δ3 is −0.4%, and FIGS. 10C and 10D correspond to a casein which Δ3 is −0.6%. In addition, FIGS. 11A and 11B correspond to acase in which Δ3 is −0.7%, and FIGS. 11C and 11D correspond to a case inwhich Δ3 is −1.0%.

FIGS. 12 and 13 are diagrams illustrating the relation of the MFD andthe cutoff wavelength with the trench position. The MFD has a value at awavelength of 1310 nm.

FIGS. 12A and 12B correspond to a case in which Δ3 is −0.4%, and FIGS.12C and 12D correspond to a case in which Δ3 is −0.6%. In addition,FIGS. 13A and 13B correspond to a case in which Δ3 is −0.7%, and FIGS.13C and 13D correspond to a case in which Δ3 is −1.0%.

In FIGS. 8A to 13D, a range surrounded by bold lines indicates a targetvalue for the threshold fiber diameter in FIGS. 10B, 10D, 11B, and 11Dand indicates the range of G.652A standard for the others. Note that thetarget value is set to be 80 μm or smaller in this example but may be asmall diameter smaller than 100 μm.

As it is clear from FIGS. 8A to 8D and 9A to 9D, the dispersioncharacteristic largely changes when the trench position is small, inother words, the trench layer is close to the core portion, but as thetrench position increases, change of the dispersion characteristicbecomes gradual and becomes stable within the standard. In addition, asit is clear from FIGS. 10A to 10D and 11A to 11D, similarly to thedispersion characteristic, when the trench position is small, change ofthe optical confinement characteristic is large but is irregular. Inaddition, as the trench position increases, change of the opticalconfinement characteristic becomes gradual, the bending loss becomeswithin the standard, and the threshold fiber diameter becomes 80 μm orsmaller.

As it is clear from FIGS. 12A to 12D and 13A to 13D, as the trenchposition increases, the MFD increases and satisfies the standard, andthereafter has a stable value. However, the cutoff wavelength has atrade-off relation with the MFD, and as the trench position increases,the cutoff wavelength increases and does not satisfy the standard. Thus,it is preferable to set the trench position so that the MFD and thecutoff wavelength both satisfy the standard.

Subsequently, simulation calculation was performed while the corediameter 2 a was 8 μm and the trench position and the trench width werechanged. Note that Δ3 was −0.4% and −0.6%, but since it was found thatΔ3 at small values of −0.7% and −1.0% does not much affect opticalproperties defined by the standard, calculation was omitted in thefollowing with consideration on manufacturing easiness of the opticalfiber.

Table 4 indicates a case in which Δ3 is −0.4%. Table 5 indicates a casein which Δ3 is −0.6%. Note that a symbol “○” is provided when G.652Astandard is satisfied, and a symbol “x” is provided when the standardnot satisfied. As indicated in Tables 4 and 5, there were a large numberof combinations of parameters with which G.652A standard was satisfiedand the threshold fiber diameter was smaller than 100 μm. In particular,there was a combination with which the threshold fiber diameter was 80μm or smaller when 43 was −0.6%. For example, the threshold fiberdiameter was 78 μm when the trench width was 1.0 and the trench positionwas 3.0.

In this manner, it is preferable that (c−b)/a is 0.2 to 1, in otherwords, (c−b) is 0.2 to 1 time larger than a when b/a is two or largerfor the trench position. In addition, it is preferable that Δ2 is equalto or smaller than −0.4%.

TABLE 4 Trench Trench Threshold fiber Zero-dispersion Dispersion CutoffMacrobending width position diameter [μm] wavelength slope MFDwavelength loss 0.2 1 >125 × ○ × ○ × 1.5 98 × ○ × ○ ○ 2 92 ○ ○ ○ ○ ○ 2.590 ○ ○ ○ ○ ○ 3 89 ○ ○ ○ ○ ○ 3.5 89 ○ ○ ○ ○ ○ 4 89 ○ ○ ○ ○ ○ 4.5 88 ○ ○ ○○ ○ 0.5 1 >125 ○ ○ × ○ × 1.5 97 × × × ○ ○ 2 90 ○ ○ ○ ○ ○ 2.5 88 ○ ○ ○ ○○ 3 87 ○ ○ ○ ○ ○ 3.5 87 ○ ○ ○ ○ ○ 4 87 ○ ○ ○ ○ ○ 4.5 86 ○ ○ ○ ○ ○ 1.01 >125 × ○ × ○ × 2 85 ○ × ○ ○ ○ 3 82 ○ ○ ○ ○ ○ 4 81 ○ ○ ○ ○ ○ Δ1 =0.36%, 2a = 8 μm, Δ3 = −0.4%

TABLE 5 Trench Trench Threshold fiber Zero-dispersion Dispersion CutoffMacrobending width position diameter [μm] wavelength slope MFDwavelength loss 0.2 1 >125 × ○ × ○ ○ 1.5 98 × ○ × ○ ○ 2 92 ○ ○ ○ ○ ○ 2.590 ○ ○ ○ ○ ○ 3 89 ○ ○ ○ ○ ○ 3.5 89 ○ ○ ○ ○ ○ 4 89 ○ ○ ○ ○ ○ 4.5 88 ○ ○ ○○ ○ 0.5 1 >125 ○ ○ × ○ × 1.5 99 × × × ○ ○ 2 88 × × ○ ○ ○ 2.5 86 ○ ○ ○ ○○ 3 85 ○ ○ ○ ○ ○ 3.5 85 ○ ○ ○ ○ ○ 4 84 ○ ○ ○ ○ ○ 4.5 83 ○ ○ ○ ○ ○ 1.01 >125 × × × ○ ○ 2 82 × × ○ ○ ○ 3 78 ○ ○ ○ ○ ○ 4 77 ○ ○ ○ × ○ Δ1 =0.36%, 2a = 8 μm, Δ3 = −0.6%

Subsequently, simulation calculation was performed while the corediameter 2 a was 7 μm, and the trench position and the trench width werechanged. FIG. 14 and Table 6 indicate the relation of the MFD with thetrench position. In comparison with the MFD in the case of no trench, itwas found that the MFD is small when the trench position is small, andthe MFD increases as the trench position increases but does not becomelarger than the MFD in the case of no trench.

TABLE 6 Trench Trench Δ3 [%] width position MFD [μm] 0 (None) 0 0 8.5−0.40 0.2 1 8 1.5 8 2 8.3 2.5 8.5 3 8.5 3.5 8.5 4 8.5 4.5 8.5 0.5 1 7.21.5 7.7 2 8.2 2.5 8.4 3 8.5 3.5 8.5 4 8.5 4.5 8.5 1 1 6.8 2 8.1 3 8.5 48.5 −0.60 0.2 1 7.7 1.5 7.8 2 8.2 2.5 8.4 3 8.5 3.5 8.5 4 8.5 4.5 8.50.5 1 6.8 1.5 7.5 2 8.1 2.5 8.4 3 8.5 3.5 8.5 4 8.5 4.5 8.5 1 1 6.4 28.1 3 8.5 4 8.5

In the range of the above simulation results, it was checked that thecore diameter 2 a of 8 μm is optimum in a case of the optical fiberaccording to the second embodiment including the trench layer.

Note that, in the case of the optical fiber according to the secondembodiment, the bending loss (macrobending loss) can be reduced becauseof the effect of the trench layer, which is favorable for application toan optical fiber that satisfies G.657 standard, which requires a smallermacrobending loss. In addition, the cutoff wavelength is allowed toshift to a longer wavelength, which is favorable for application to anoptical fiber that satisfies G.654 standard.

Subsequently, a parameter combination having a most favorablecharacteristic among the step-type parameter combinations listed inTable 2 and a parameter combination having a most favorablecharacteristic among the trench-type parameter combinations listed inTable 5 are listed as optimum parameter combinations in Table 7. Inaddition, Table 8 lists G.652A standard and optical properties of anoptical fiber in the case of Table 7. For the step type, 80 μm wasobtained as the threshold fiber diameter through optimization of therefractive index profile of the core portion. In addition, for thetrench type, 78 μm was obtained as the threshold fiber diameter becauseof the effect of the trench layer.

TABLE 7 Item Δ1 Δ3 2a Unit [%] [%] b/a c/a [μm] Step 0.37 — — — 9.0 typeTrench 0.36 −0.60 3 4 8.0 type

TABLE 8 Dispersion slope Cutoff Macrobending Threshold ItemZero-dispersion [ps/nm²/km] MFD wavelength loss fiber Unit wavelength @zero-dispersion [μm] [nm] [dB/m @ 60 mm] diameter G.652 A [nm]wavelength @ 1310 nm @ 22 m @ 1550 nm [μm] Standard 1300-1324 ≤0.0928.6-9.5 ≤1260 ≤5.3E−3  (<100) Step type 1304 0.090 9.1 1254 3.3E−7 80Trench type 1317 0.088 8.8 1252 6.1E−6 78

Subsequently, Table 9 lists G.657A2 standard and the optical propertiesof an optical fiber in the case of Table 7. In the case of G.657Astandard, the macrobending loss is defined to be a value at a wavelengthof 1550 nm when bending is made at a diameter of 20 mm, which means astandard having more rigorous requirements than those of G.652. Asunderstood from Table 9, G.657A2 standard was satisfied in the case ofthe trench-type, and superiority of the trench type was confirmed.

TABLE 9 Dispersion slope Cutoff Macrobending Threshold ItemZero-dispersion [ps/nm²/km] MFD wavelength loss fiber Unit wavelength @zero-dispersion [μm] [nm] [dB/m @ 20 mm] diameter G.657 A2 [nm]wavelength @ 1310 nm @ 22 m @ 1550 nm [μm] Standard 1300-1324 ≤0.0928.6-9.2 ≤1260 ≤1.59 (<100) Step type 1304 0.090 9.1 1254 6.82 80 Trenchtype 1317 0.088 8.8 1252 0.26 78

Subsequently, how the optical properties of an optical fiber are changedwhen the optimum trench-type parameters listed in Table 5 are changedwill be described below. Table 10 lists parameter combinations ofTrenches 2 to 15 in which Δ1, Δ3, b/a, c/a, and 2 a are changed with theoptimum trench-type parameters listed in Table 5 as reference values(Trench 1). Note that, in the table, a symbol “-” indicates a value sameas a reference value. Table 11 lists optical properties of opticalfibers having parameters of G.652A standard and Trenches 1 to 15.

TABLE 10 Item Δ1 Δ3 2a Unit [%] [%] b/a c/a [μm] Trench 1 0.36 −0.60 3 48.0 (reference) Trench 2 0.35 — — — — Trench 3 0.37 — — — — Trench 4 —−0.55 — — — Trench 5 — −0.63 — — — Trench 6 — −0.65 — — — Trench 7 — —2.8 — — Trench 8 — — 2.9 — — Trench 9 — — 3.2 — — Trench 10 — — — 3.8 —Trench 11 — — — 4.05 — Trench 12 — — — 4.2 — Trench 13 — — — — 7.8Trench 14 — — — — 8.1 Trench 15 — — — — 8.2

TABLE 11 Dispersion slope Cutoff Macrobending Threshold ItemZero-dispersion [ps/nm²/km] MFD wavelength loss fiber Unit wavelength @zero-dispersion [μm] [nm] [dB/m @ 60 mm] diameter G.652 A [nm]wavelength @ 1310 nm @ 22 m @ 1550 nm [μm] Standard 1300-1324 ≤0.0928.6-9.5 ≤1260 ≤5.3E−3  (<100) Trench 1 1317 0.088 8.8 1252 6.1E−6 78Trench 2 1317 0.088 8.9 1239 2.1E−5 81 Trench 3 1316 0.087 8.8 12601.7E−6 76 Trench 4 1316 0.087 8.7 1243 7.6E−6 79 Trench 5 1317 0.087 8.81260 5.3E−6 78 Trench 6 1317 0.088 8.8 1264 4.9E−6 77 Trench 7 13150.089 8.8 1287 3.3E−6 76 Trench 8 1316 0.088 8.8 1269 4.5E−6 77 Trench 91318 0.087 8.8 1226 1.1E−5 81 Trench 10 1319 0.087 8.8 1217 1.2E−5 81Trench 11 1318 0.087 8.8 1264 5.1E−6 77 Trench 12 1319 0.087 8.8 13003.1E−6 75 Trench 13 1320 0.087 8.8 1221 1.8E−5 81 Trench 14 1315 0.0888.9 1268 3.6E−6 77 Trench 15 1314 0.088 8.9 1284 2.1E−6 76

FIG. 15 is a diagram illustrating change of the threshold fiber diameterwhen parameters are changed. As it is clear from the results of Tables10 and 11, it can be understood that the threshold fiber diameter can befurther reduced beyond 78 μm by, for example, changing parameters in thedirections of change as indicated for Trenches 3, 5, 8, 11, and 14 (thedirections of increase (UP) and decrease (DOWN) illustrated with solidline arrows in FIG. 15 ) but the cutoff wavelength shifts to thelong-wavelength side. Trenches 3, 5, 8, 11, and 14 correspond tocharacteristics when the cutoff wavelength is near 1260 nm, which is astandard upper limit, but it can be understood that there is hardly roomfor improvement since the cutoff wavelength of Trench 1 as a referenceis 1252 nm, which is close to the standard upper limit. In addition, asillustrated in FIG. 15 , largest improvement was observed for thethreshold fiber diameter when Δ1 was increased. It can be understoodfrom this result as well that it is important to have an optimumstructure for the core portion. For example, such characteristicimprovement due to application of the trench type that G.652A standardsatisfied and the threshold fiber diameter is 76 μm was achieved withTrench 3. Note that, in a case in which the present reference was used,the cutoff wavelength was a restriction factor of characteristicimprovement when any parameter was changed, but it is thought that therestriction factor of characteristic improvement can be anotherparameter depending on reference selection, and thus it is preferable todetermine the restriction factor when performing optimization like thepresent method.

Note that, in the embodiments, the modification, the calculationresults, and the example described above, an optical fiber has astep-type or trench-type refractive index profile, but these areexemplary preferred examples. In other words, the present disclosure isnot limited thereto but includes a case in which, for example, theeffects of the present disclosure are obtained with the step type or thetrench type by using parameters different from those above-described. Inaddition, the present disclosure also includes, other than the step typeand the trench type, a case in which a small-diameter optical fiberhaving high versatile characteristics is achieved employing a coreportion having the above-described optimum structure. For example, thepresent disclosure certainly includes an optical fiber having what iscalled a W-type refractive index profile in which the trench layer isadjacent to the core portion in the trench type as described in thesecond embodiment. In this case, it is preferable to design the trenchlayer so that the trench layer provides change in an allowable range tooptical properties achieved by the core portion having the optimumstructure. In addition, as another embodiment, Δ+ layer may be providedat a position favorable for the core portion having the optimumstructure. Here, the Δ+ layer is a layer surrounding the core portionand having a positive relative refractive-index difference with respectto the reference refractive index region of the cladding portion. Withsuch an optical fiber according to the present disclosure, it ispossible to excellently achieve a small-diameter optical fiber having areduced leakage loss and satisfying various reference standards.

Note that the present disclosure is not limited by the above-describedembodiments. The present disclosure includes a configuration as acombination of above-described components as appropriate. In addition,further effects and modifications can be easily derived by the skilledperson in the art. Thus, a broader aspect of the present disclosure isnot limited to the above-described embodiments but can have variouschanges.

Industrial Applicability

An optical fiber according to the present disclosure can be excellentlyused for signal transmission.

According to the present disclosure, an effect of achieving asmall-diameter optical fiber having a reduced leakage loss is obtained.

Although the disclosure has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

The invention claimed is:
 1. An optical fiber comprising: a core portionmade of glass; and a cladding portion made of glass, having a refractiveindex lower than the refractive index of the core portion, andpositioned on an outer periphery of the core portion, wherein thecladding portion has an outer diameter smaller than 100 μm, and the coreportion has a relative refractive-index difference of 0.32% to 0.40%with respect to the cladding portion, wherein the cladding portionincludes a trench layer adjacent to the core portion, and the trenchlayer has a relative refractive-index difference equal to or larger than−0.1% and smaller than 0% with respect to the cladding portion exceptfor the trench layer.
 2. The optical fiber according to claim 1, whereinthe core portion has a core diameter of 7 μm to 10 μm.
 3. The opticalfiber according to claim 1, wherein the core portion has a relativerefractive-index difference of 0.335% to 0.375%, and the core portionhas a core diameter of 8.2 μm to 9.2 μm.
 4. The optical fiber accordingto claim 1, wherein the core portion has a relative refractive-indexdifference of 0.37% to 0.395%, and the core portion has a core diameterof 7.6 μm to 8.3 μm.
 5. The optical fiber according to claim 1, whereinthe cladding portion includes an adjacent region that is adjacent to theouter periphery of the core portion, and a non-adjacent regionpositioned on an outer periphery of the adjacent region, the adjacentregion being interposed between the non-adjacent region and the coreportion, and the adjacent region is made of pure silica glass.
 6. Theoptical fiber according to claim 5, wherein the non-adjacent regionincludes a trench layer having a refractive index lower than therefractive index of the adjacent region.
 7. The optical fiber accordingto claim 6, wherein b/a is equal to or larger than two where 2 arepresents a core diameter of the core portion and 2 b represents aninner diameter of the trench layer.
 8. The optical fiber according toclaim 6, wherein b/a is equal to or larger than three where 2 arepresents a core diameter of the core portion and 2 b represents aninner diameter of the trench layer.
 9. The optical fiber according toclaim 6, wherein the width of the trench layer, which is given as c−b,is 0.2 to 1 time larger than a where 2 a represents a core diameter ofthe core portion, 2 b represents an inner diameter of the trench layer,and 2 c represents an outer diameter of the trench layer.
 10. Theoptical fiber according to claim 6, wherein the trench layer has arelative refractive-index difference equal to or smaller than −0.4% withrespect to the adjacent region.
 11. The optical fiber according to claim1, wherein the optical fiber has a mode field diameter of 8.6 μm to 9.5μm at a wavelength of 1310 nm.
 12. The optical fiber according to claim1, wherein the optical fiber has a bending loss equal to or smaller than5.3×10−3 dB/m at a wavelength of 1550 nm when bent at a diameter of 60mm.
 13. The optical fiber according to claim 1, wherein the opticalfiber has a zero-dispersion wavelength of 1300 nm to 1324 nm and adispersion slope equal to or smaller than 0.092 ps/nm²/km at thezero-dispersion wavelength.
 14. The optical fiber according to claim 1,wherein the optical fiber has a cable cutoff wavelength equal to orshorter than 1260 nm.
 15. The optical fiber according to claim 1,wherein the optical fiber has a mode field diameter equal to or largerthan 9.5 μm at a wavelength of 1550 nm.
 16. The optical fiber accordingto claim 1, wherein the optical fiber has a cable cutoff wavelengthequal to or shorter than 1530 nm.